1. Field of the Invention
The present invention relates to a magnetic recording element, magnetic recording device and recording method of information, and more particularly to a magnetic recording element capable of controlling the magnetization direction of a magnetic substance by use of the spin-transfer torque so as to record data.
2. Description of the Related Art
Information recording apparatuses have been used to satisfy a variety of demands such as increased capacity, high speed, excellent durability, low cost as an existence which supports wide and highly information age society and leads it in recent years and technology for improving such features has been demanded. Of these components, the magnetic recording device using magnetic moment of a ferromagnetic substance has been used as, for example, a hard disk drive currently and recently, use as a magnetic random access memory (MRAM) having both high speed and nonvolatile performances has been proposed.
However, a demand for high density memory has reached 100 nm to several tens nm or a shorter scale as a unit cell for storing 1-bit data, so that technical barrier is appearing in data writing style. That is, the smaller the memory cell is, the more the current necessary to generate magnetic field for writing in current magnetic field writing style used in hard disk drive or MRAM. Current magnetic field writing style also cannot prevent the cross talk to adjoining cells.
Magnetization switching by use of the spin-transfer torque, which is actually verified in F. J. Albert et al., Appl. Phys. Lett., vol. 77, 3809 (2000), pp. 77 recently, is expected as a new magnetic recording method capable of solving the problems of the current magnetic field writing style.
According to this phenomenon, when flow of spin-polarized electron passes a magnetic substance whose magnetization direction is directed in antiparallel to the electron, spin angular momentum of conduction electron is applied and transmitted to the magnetization of the magnetic substance to generate a torque to switch the magnetization. This phenomenon enables a more direct operation to be applied to a nano scale magnetic substance as compared with magnetization switching by current magnetic field. Thus, no cross talk occurs and high-speed magnetization switching can be expected. Additionally, there is an advantage that the current necessary for writing decreases as the cell size decreases.
However, it is hard to say that the most commonly used magnetization switching by use of the spin-transfer torque sufficiently demonstrates its potential. The current necessary for magnetization switching is extremely large, from 10 mA to several mA even in case where the cell size is 100 nm to several tens nm, so that the device may be damaged and this cannot satisfy the demand for lowered power consumption. Furthermore, it is reported that magnetization switching takes approximately few nano second (see, for example, R. H. Koch et al., Phys. Rev. Lett., vol. 92, 088302, (2004)). A higher speed is demanded for application of exchange of information with a high-speed micro processor.
A conventional magnetization switching method by use of the spin-transfer torque will be described. First, the magnetization switching by use of the spin-transfer torque called a conventional method 1 in this specification will be described. The conventional method 1 shown in FIG. 48 employs a magnetic recording element using a lamination structure constituted of fixed layer FP, intermediate layer S, and free layer FF as its basic structure. The magnetization direction of the free layer FF is parallel to or antiparallel to the magnetization direction of the fixed layer FP. A flow of a current whose strength is higher than a critical value Jc is passed in the vertical direction to the film face of this device (face which respective laid films face) to switch the magnetization. A current 1.5 times larger than the critical value Jc was passed to such a device as simulation. In this simulation, the initial angle of magnetization direction of the free layer FF with respect to the magnetization direction of the fixed layer FP was set to 5 degrees in the film face plane. As a result, time taken for magnetization switching was approximately 7 ns. The smaller the current or the initial angle, the more the switching time takes.
As the magnetization switching method by use of the spin-transfer torque, another method (called conventional method 2) is proposed (Jpn. Pat. Appln. KOKAI Publication No. 2002-261352). The conventional method 2 is carried out by introducing a current which is spin-polarized in a direction perpendicular to the magnetization direction of the free layer. FIG. 49 shows schematically the sectional structure of a recording device used for the conventional method 2. This device has a lamination structure constituted of spin supply layer FPW, intermediate layer SW, free layer FF, intermediate layer SR and fixed layer FPR and the magnetization of the spin supply layer FPW is perpendicular to the film face.
As the current keeps supplied to such a device, the magnetization of the free layer FF executes precessional motion. If the injection of the current is stopped, the magnetization stops its processional motion and approaches to the final state which is dependent on the time-width of the supplying current. That is, if an introduction of a current is stopped at a time t1, the magnetization is switched from the initial condition to the opposite one. A. D. Kent et al., Appl. Phys. Lett., vol. 84, 3897 (2004) states that magnetization switching is possible at 50 ps by using the substantially same method. However, a slight shift of the stop timing of the current from t1 causes the magnetization direction to return to the initial state. Thus, in this method, accurate control of the time for supplying current is required as well as the suppression of the variation of the magnetization direction in each device. Further, because such magnetization switching depends on the initial state, the read-before-write is necessary.
In summary, the current technology of the magnetization switching by use of the spin-transfer is not sufficient in light of current consumption and high-speed characteristics. Although the conventional method 2 is principally capable of high speed magnetization switching, it is needed to suppress the variation of magnetization direction among elements and accurately control the time for the current supply.